In the recent drive for higher integration and operating speeds in LSI devices, it is desired to miniaturize the pattern rule. Great efforts have been devoted for the development of the micropatterning technology using deep-ultraviolet (deep-UV) or vacuum-ultraviolet (VUV) lithography. The photolithography using KrF excimer laser (wavelength 248 nm) as the light source has already established the main role in the commercial manufacture of semiconductor devices. The lithography using ArF excimer laser (wavelength 193 nm) is at the start of its application to commercial fabrication by the advanced micropatterning technology. For the ArF excimer laser lithography, however, partly because the succeeding technology has not been established, it is strongly desired to improve the performance of resist material in order to gain a further stretch of resolution. Efforts are also made to develop the immersion lithography process that intends to gain a stretch of resolution by interposing a high refractive index liquid between a resist coating film and a projection lens. There is a need for a resist material meeting such requirements.
For resist materials adapted to the ArF excimer laser lithography, polyacrylic or polymethacrylic acid derivatives and polymers comprising cycloaliphatic compounds in the backbone are under investigation. In either case, the basic concept is that some or all of alkali soluble sites of an alkali soluble resin are protected with acid labile groups. The overall performance of resist material is adjusted by a choice of an adequate protecting group from among numerous acid labile protecting groups.
Exemplary acid labile protecting groups include tert-butoxycarbonyl (JP-B H02-27660), tert-butyl (JP-A S62-115440 and J. Photopolym. Sci. Technol. 7 [3], 507 (1994)), 2-tetrahydropyranyl (JP-A H02-80515 and JP-A H05-88367), 1-ethoxyethyl (JP-A H02-19847 and JP-A H04-215661), and adamantane structure-containing alkyl (JP-A H09-73173 and JP-A H15-64134). While it is desired to achieve a finer pattern rule, none of these acid labile protecting groups are deemed to exert satisfactory performance.
More particularly, since tert-butoxycarbonyl and tert-butyl are least reactive with acids, a substantial quantity of energy radiation must be irradiated to generate a sufficient amount of acid in order to establish a difference in dissolution rate before and after exposure. If a photoacid generator of the strong acid type is used, the exposure dose can be reduced to a relatively low level because reaction can proceed with a small amount of acid generated. In this event, however, the deactivation of the generated acid by air-borne basic substances has a relatively large influence, giving rise to such problems as T-top pattern profile. On the other hand, 2-tetrahydropyranyl and 1-ethoxyethyl are reactive with acids and advantageous from the standpoint of resolution. Where these groups are used as protective groups for carboxylic acid, thermal stability is poor and a concern about shelf stability is left.
The resist materials described above commonly suffer from noticeable line density dependency (or optical proximity effect) in that when a pattern including a high density (or grouped) region and a low density (or isolated) region is to be transferred, exposure at an identical dose fails to produce the desired pattern which is satisfactory in both the regions. Generally speaking, in the ArF excimer laser lithography, the acid generated upon exposure triggers deprotection reaction on the base resin which proceeds during heat treatment following exposure (post-exposure bake or PEB). Migration of the acid occurs during PEB. Since chemically amplified resist materials depend on the function of the acid to act as a catalyst to promote deprotection reaction, moderate acid migration is necessary. However, acid migration degrades an optical image, indicating that excess acid migration detracts from resolution. To comply with the outstanding demands for a further size reduction in the ArF excimer laser lithography and a higher resolution due to a good command of the immersion lithography, there is a need for a resist material featuring controlled acid migration and higher resolution capability.